Finding of Neoproterozoic (∼775 Ma) magmatism recorded in metamorphic complexes from the North Qilian orogen: Evidence from SHRIMP zircon U-Pb dating

In order to constrain the formation time of high-grade metamorphic rocks in the Qilian Mountains, U-Pb zircon dating was carried out by using LA-ICPMS technique for a paragneiss of the Hualong Group in the Qilian Mountains basement series and a weakly foliated granite that intruds into the Hualong Group. Zircons from the paragneiss consist dominantly of detrital magma zircons with round or sub-round shape. They have 207 Pb/ 206 Pb ages mostly ranging from 880 to 900 Ma, with a weighted mean age of 891 ±9 Ma, which is interpreted as the magma crystallization age of its igneous provenance and can be taken as a lower age limit for the Hualong Group. Magma crystallization age for the weak-foliated granite is 875±8 Ma, which can be taken as an upper age limit for the Hualong Group. Accordingly, the formation time of the Hualong Group is constrained at sometime between 875 and 891 Ma. A few zircons from both paragneiss and weak-foliated granite display old inherited ages of 1000 to 1700 Ma and young metamorphic ages of Early Paleozoic. The zircon age distribution pattern confirms that the Qilian Mountains and the northern margin of Qaidam Basin had a united basement, with geotectonic affinity to the Yangtze Block. The results also reveal that sediments of the Hualong Group formed by rapid accumulation due to rapid crustal uplift-erosion. This process may result from intensive Neoproterozoic orogenesis due to assembly of the suppercontinent Rodinia.

Section: Formation Time Of the Hualong Group And Its Tectonic Implicasupporting

confidence: 81%

U-Pb zircon dating constraints on formation time of Qilian high-grade metamorphic rock and its tectonic implications

Zhang

Liu

2007

“…These protolith and subduction ages are possibly reflected by the formation and overgrowth ages of zircon in mylonized granitic gneisses from the Gehai area in the Qilian Block (790±12 Ma and 472±6 Ma, respectively). From the metamorphic complex in the middle and central parts of the North Qilian suture zone, Tseng et al [65] discovered a granite and a tonalite with intrusive ages of ~775 Ma, which are comparable to the protolith ages in the Dabie-Sulu metamorphic terrane. As proposed for the Sulu-Dabie orogenic belt, the model involving a passive-margin accretionary wedge deformed during continent subduction [66] may also be applied to the Qilian Block.…”

Section: Evolution Of the Qilian Blockmentioning

confidence: 97%

“…The late Mesoproterozoic to Neoproterozoic granitic gneisses on the Qilian Block might also be accretionary wedge on passive-margin but have different metamorphic ages. Therefore, the Qilian region and Dabie-Sulu orogenic belt are of similar tectonic settings forming during Caledonian and Indosinian periods, respectively [44,65] .…”

Section: Evolution Of the Qilian Blockmentioning

confidence: 99%

SHRIMP U-Pb geochronology of the zircons from the Precambrian basement of the Qilian Block and its geological significances

Tung¹,

Yang²,

Yang³

et al. 2007

Self Cite

153

Origin and tectonic evolution of the Qilian Precambrian basement on NW China were investigated using zircon U-Pb ages with collaborating stratigraphic and paleontological evidence. Zircon grains were separated from two schists, two granitic gneisses and one mylonized gneiss and dated with SHRIMP. Seventy percent of sixty-one detrital zircon ages from two schists ranges from 0.88 Ga to 3.09 Ga, mostly within 1.0 Ga to 1.8 Ga with a peak at 1.6 Ga to 1.8 Ga, and twenty percent varies from 2.0 Ga to 2.5 Ga. A few falls in the Archean and Neoproterozoic periods. The two granitic gneisses were dated 930±8 Ma and 918±14 Ma, whereas the mylonized granitic gneiss was dated 790±12 Ma. These ages represent two periods of magmatisms, which can be correlated with the early and late stages of magmatisms associated with the Jinningian movement on the Yangtze Blocks.The results from this and previous studies indicate that the ages of the Precambrian detrital zircons from the Qilian Block are widely distributed in the Proterozoic era, distinct from the North China Block which was stable in the Neo-Mesoproterozoic era. By contrast, the age histograms of the detrital zircons from the Qilian Block is similar to those from Precambrian basement of the Yangtze Craton. Therefore, it is suggested that the Qilian Block had a strong affinity toward the Yangtze Craton and might belong to the supercontinent Gondwana in the Neoproterozoic time. This inference is supported by Nd model age (T DM ), stratigraphic, and paleontological evidence. It is further considered that the Qilian Block was rifted from the supercontinent Gondwana during late Sinian to form an isolated continent in the Proto-Tethyan Ocean, moving towards the Alaxa Block in the North China Craton. The part of Proto-Tethyan Ocean between the Qilian and Alaxa Blocks should correspond to the so-called Paleo-Qilian Ocean. Following the closure of the Paleo-Qilian Ocean in the early Paleozoic, the Qilian Block collided with the Alaxa Block to form the North Qilian Orogenic Belt. Based on this tectonic explanation, the North Qilian ophiolites should represent parts of lithosphere from the Proto-Tethyan Ocean. Lithological and geochronological evidence also indicates that the Qilian Block underwent continental reactivation possibly induced by the deep northward subduction of the North Qaidam Block in early Paleozoic time.

“…Following this rationale, it is inferred that the analyzed zircon grains were crystallized from melts produced by partial melting during obduction of oceanic crust or of exotic crust origin. Because the continental crust in the North Qilian orogenic belt are of Precambrian ages [31][32][33] , the early Ordovician zircon in the Dongcaohe ophiolite are not crustal relics but formed during tectonic emplacement indicating the existence of an ocean basin older than early Ordovician ages most possibly of middle-late Cambrian period.…”

Section: Zircon Shrimp U-pb Datingmentioning

confidence: 99%

The Dongcaohe ophiolite from the North Qilian Mountains: A fossil oceanic crust of the Paleo-Qilian ocean

Tseng¹,

Yang²,

Yang³

et al. 2007

Self Cite

The Dongcaohe ophiolite, located at the south of the North Qilian subduction complexes, is a tectonic block with an exposed area of about 3 km×6 km. It consists of an intrusive section overlain by an extrusive section. The lower part of the intrusive section consists of cyclic layers of cumulate dunites, troctolites, anorthosites, anorthositic gabbros, and gabbros with small discordant dunite and troctolite bodies. This layered sequence grades upward to isotropic gabbros and gabbronorites, which are overlain by the extrusive sequence of diabasic sheeted dikes and basaltic lavas. The overall mineral crystallization sequence was olivine±Cr-spinel, plagioclase, clinopyroxene, orthopyroxene, and Fe-Ti oxides. The Cr-spinel (Mg#: 42-66, Cr#: 41-57) in these layered cumulates and present-day abyssal peridotites have similar compositions. Also, the compositional variations of the plagioclase and clinopyroxene in the intrusive section reflect crystallization from melts compositionally similar to the present-day ocean basalts. Moreover, the rare earth element (REE) and multi-element distribution patterns of the intrusive and extrusive lithologies in the Dongcaohe ophiolite are consistent with crystallization of mid-ocean ridge basalts. The zircon grains separated from the gabbronorite have an SHRIMP average 206 Pb/ 238 U weighted age of 497 ± 7 Ma, which is considered as the tectonic emplacement age of the Dongcaohe ophiolite. The field occurrence, mineral and whole-rock compositions indicate that the Dongcaohe ophiolite represents a well-persevered oceanic crustal fragment composed of a complete oceanic crustal section of layered cumulates at bottom upgrading through isotropic cumulates to sheeted dikes and lava flows. ophiolite, North Qilian, oceanic crust, DongcaoheOphiolites are sections of the oceanic crust and subjacent upper mantle thrust up on continental and island arc margins [1,2] . Those in China usually occur as dismembered ophiolitic fragments. Well-preserved bottom-totop lithological sequence of peridotite-gabbro-sheeted dike-basalt that characterizes oceanic lithosphere is rare [2] . We, on the basis of lithological assemblages, petrographic textures, and geochemical characteristics, recently identified a section of entire oceanic crust at Dongcaohe on the south flank of the North Qilian orogenic belt and referred it to as the "Dongcaohe ophiolite".In this study, field occurrence, geochronology, mineral and bulk compositions were used to address the tectonic environments associated with the Dongcaohe ophiolite and their geological implications.